The present invention relates to a method of bonding multiple metal plates, an apparatus therefor, and a hot strip mill; particularly to a method of bonding hot rolled strips which makes it possible to bond the hot rolled strips in a short time and perform continuous rolling in case where the hot rolled strips are rolled by a set of coarse rolling mills and a set of finish rolling mills.
There has been a keen desire for performing finish rolling continuously in a hot strip mill for metal plates so as to improve productivity and quality as well as to realize automated operation. A key technique to this continuous finish rolling is how to bond hot rolled strips (hereinafter called sheet bars). In case of cold rolled strips, continuous rolling operation associated with bonding with sufficient strength, such as by welding, is possible because the cold rolled strips are thinner. In case of hot rolled strips, however, rolling speed (feeding speed of rolled strips) is faster than in a cold strip mill and accordingly bonding by means of welding is difficult, because the hot rolled strips are thicker and because the rolling operation must be completed before the temperature of the rolled strips becomes lower.
Up to now, there have been a number of proposed methods of bonding sheet bars, including an electric heating method, a gas heating method, a melt-chopping method, and a friction method, but each has a disadvantage that bonding process takes too much time. Since the rolling speed of the hot rolled strips is faster as described above, a running type bonding machine cannot be realized unless bonding the sheet bars is completed in a short time, because the traveling distance of the machine becomes longer. If a fixed type bonding machine is employed, a huge looper for accumulating sheet bars becomes necessary, because sheet bars are normally as thick as 20 to 50 mm. A conventional bonding process of the sheet bars took 20 to 30 seconds, at the shortest, including preparation for bonding and removal of roll-over and burrs caused by pressing.
Known as a technique for shortening the bonding time of hot rolled strips is one disclosed in Japanese Patent Application Laid-Open Publication No. HEI 9-174117 (1997). According to this technique, a preceding sheet bar and a following sheet bar are overlapped and then the two sheet bars are sheared at the same time, during which clean surfaces are brought into direct contact with each other so as to achieve strong metal bonding.